The alkylation of aromatics with olefins to produce monoalkyl aromatics is a well developed art which is practiced commercially in large industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene which is subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene) which is subsequently used in the production of phenol and acetone. Those skilled in the art are therefore familiar with the general design and operation of such alkylation processes.
Commercial cumene processes usually run with a molar ratio of phenyl groups per propyl group in the alkylation reactor that is greater than the stoichiometric ratio of 1:1 in order to avoid forming by-products. A high molar ratio primarily acts as a heat sink and controls the exothermic temperature rise in the reactor, thus reducing n-propylbenzene formation which is promoted at high temperature. Current specifications for n-propylbenzene in cumene product streams are in the range of 250 to 300 wppm, where wppm refers to parts of n-propylbenzene by weight per million parts of cumene by weight. A high molar ratio also reduces formation of diisopropylbenzene (DIPB) and triisopropylbenzene (TIPB) by polyalkylation and of propylene dimer and propylene trimer by oligomerization.
Prior art methods are known for controlling the temperature rise by using the excess benzene in the alkylation reactor effluent. The alkylation reactor effluent contains excess or unreacted benzene, of course, primarily because the molar ratio in the alkylation reactor is greater than stoichiometric. One prior art method distills this excess benzene from the reactor effluent and recycles a benzene stream to the inlet of the alkylation reactor, or to one or more of the reactor beds in the case of a multibed reactor. Another prior art method recycles some of the reactor effluent without separation to the inlet of the alkylation reactor. Both prior art methods leave room for improvement. In the former method, distilling benzene from the reactor effluent wastefully consumes utilities used in vaporizing and condensing benzene. In the latter method, recycling reactor effluent to the alkylation reactor inlet requires a large quantity of reactor effluent recycle to all of the reactor beds.
Not only must excessively high temperatures in the alkylation reactor be prevented, but excessively low temperatures must be avoided as well. For solid cumene alkylation catalysts, low temperatures accelerate catalyst deactivation by allowing heavy polyalkyl aromatics to deposit in the catalyst pores. Rapid deactivation shortens the catalyst life and necessitates more frequent shutdowns for the catalyst to be regenerated or replaced, thereby hurting the profitability of the cumene process.
Thus, a method is sought for preventing excessive temperatures, both high and low, in a multibed cumene alkylation reactor while efficiently using the excess benzene in the alkylation reactor effluent.